As a damping force control apparatus of a suspension, for example, the technique described in JP 10-913A (Reference 1) is known.
The damping force control apparatus according to the technique includes opening determination means for determining an orifice opening degree of a damper so as to control a damping force. The opening determination means determines the orifice opening degree (damping force control value) based on a vehicle body speed (absolute speed described in Reference 1) and a relative speed of a vehicle wheel with respect to a vehicle body. Here, each of the vehicle body speed and the relative speed is calculated based on the output (acceleration) obtained from an acceleration sensor of the vehicle body.
The relative speed is estimated by applying the Kalman filter theory to a modeled damper. In applying the Kalman filter theory, the damping force of the damper is classified as a linear component and a nonlinear component.
In addition, in JP 2010-58541A (Reference 2), an output deviation corresponding to the difference between the above-suspension speed (observation output) of the vehicle and the estimated above-suspension speed (estimated observation output) from a vehicle approximate model of an observer for estimating an actual vehicle model state quantity is obtained. Based on the output deviation, a first observer gain of the observer for actual damping force estimation calculates a dynamic characteristic compensation signal, and a second observer gain of an observer for actual vehicle model state quantity estimation calculates a vehicle model compensation signal. The dynamic characteristic compensation signal is input to dynamic characteristic imparting means of the observer for the actual vehicle model state quantity estimation and is used for adjusting setting contents of the dynamic characteristic imparting means.
However, in the estimation of the relative speed according to the related art described above, the estimated relative speed is greatly diverged from the actual relative speed and as a result, the orifice opening degree (damping force control value) may not be calculated as an appropriate value, that is, a value suitable for suppressing vibration of the vehicle body.